Many of the smaller lakes have been transformed into peatbogs (raised mires) with peat layers of various depths and diverse vegetation cover. Thickness of glacial formations – clay, sand and gravel locally exceeds 150 m. The Lake Wigry of 2.163 ha surface and maximal depth of 73 m remains in close hydrological contact with most of the smaller lakes. Pierty is the second larger lake of a surface of 228 ha and 38 m deep. The lakes display a variety of features of trophic status of water. Separate group constitute mid-forest dystrophic lakes whose dark water is highly saturated with organic substances. The dystrophic lakes have been developed in the process of lake overgrowing.